"It's very common," says Jess Hanham casually, when asked how often he finds suspected unexploded bombs. Mr Hanham is a co-founder of Spectrum Offshore, a marine survey firm that does a lot of work in the Thames Estuary. His firm undertakes all sorts of marine surveying, but working on sites for new offshore wind farms has become a big business for him. Work in the Thames Estuary, and other areas that were the targets of bombing in World War 2, are likely to involve picking up signals of unexploded munitions. "You can find a significant amount of contacts that need further investigation and for a wind farm that will be established in the initial pre-engineering survey," he says. With that information project managers can decide whether to place turbines and other equipment a safe distance from the suspected bombs, or have them blown up by a specialist firm. At the moment marine surveying is done by teams who go out on boats, collect the data and bring it back for analysis. Sometimes that will involve a relatively small vessel with two crew members, a surveyor and his kit. But bigger inspection projects further out to sea can involve much larger boats, with dozens of crew members, costing in the region of £100,000 per day. The sensor equipment varies according to the job. Sometimes it might be a sonar array towed behind the boat, for other jobs it might be an underwater unmanned vehicle, which can be controlled by surveyors on the surface. Bad weather can disrupt the work and make life uncomfortable. "I've been at sea in force nine and force 10 gales and they're not nice places to work," says Brian Allen, chief executive of Rovco.